1. Field of the Invention
This invention relates to a printing head for use in a printer, a facsimile machine, a plotter, a word processor and so on, and more particularly to an inkjet printing head.
2. Description of the Related Art
A number of inkjet printers using inkjet printing heads have been in wide use. In such a printer, the inkjet printing head emits a steady stream of ink drops onto a printing medium without coming into contact with the medium. The inkjet printer does not require the printing medium such as paper whose quality is strictly technically controlled, and is advantageous in that it can assure high quality printing.
An example of the inkjet printing head comprises piezoelectric elements, a plurality of nozzles, pressure chambers, and a thin diaphragm. The nozzles are in communication with the pressure chambers as temporary ink retainers. The pressure chambers are open at their upper portions. The diaphragm made from glass or resin covers the open portions of the pressure chambers. The piezoelectric elements are disposed on the diaphragm in such a manner as to correspond to the pressure chambers on a one-to-one basis. To emit ink via the nozzles, a voltage is applied to piezoelectric elements. The piezoelectric elements are flexed, thereby causing associated portions of the diaphragm to be flexed too. The capacity of the pressure chambers is reduced so as to emit ink via nozzles. When the voltage is interrupted, the piezoelectric elements and the diaphragm return to their normal state. Thereafter, the pressure chambers suck in ink from ink supplies, thereby preparing for succeeding ink emission.
Bubble jet printers have also been popular. In this type of printer, ink is heated by a heater. The heated ink generates bubbles, so that ink is emitted in drops utilizing a vapor pressure of the bubbles. The bubble jet printer does not require large pressure chambers compared with the inkjet printer which uses piezoelectric elements to change the capacity of the pressure chambers, and is advantageous in that nozzles can be integrated in a very closely packed manner.
A printing head for a color printer will be described hereinafter. Assume that a bubble jet printer comprises an inkjet printing head 100 as shown in FIG. 1. The inkjet printing head 100 includes a plurality of nozzle rows 102, each of which has a plurality of nozzles 101. The nozzles 101 are aligned in a sub-scanning direction in which a printing medium such as paper, not shown, is moved. Referring to FIG. 1, four nozzle rows 102 are juxtaposed in a main scanning direction in which the inkjet printing head 100 moves. The nozzles in the four rows 102 emit ink of different colors such as yellow (Y), magenta (M), cyan (C) and black (BK) colors, respectively. These four colors are overlaid so as to produce printed images in a variety of colors.
FIG. 2 shows the arrangement of nozzles used for an inkjet printing head 200 of the bubble jet printer or an inkjet printer including piezoelectric elements. The inkjet printing head 200 has a nozzle row 103 constituted by a plurality of nozzles 101. The nozzle row 103 is inclined in both the sub- and main scanning directions, and is divided into four blocks a-d. The nozzles 101 in these blocks a-d emit different colors (Y, M, C and BK), respectively, so as to produce an image having various colors.
Quick drying ink should be used when the color printing is performed by arranging the nozzle rows 102 and nozzle blocks a-d in the main scanning direction. This is because ink of a first color is emitted, and ink of a second color is emitted immediately after the first one, and these two colors will be overlaid. In such a case, ink containing a penetrant is used so as to penetrate into the sheet and become dry quickly.
With the inkjet printing head using the piezoelectric elements, it is very difficult to arrange nozzles in a packed manner as with the bubble jet printing head. As shown in FIG. 2, the nozzles 101 are inclined in the sub-scanning direction so as to improve dot pitches. However, this inevitably enlarges the inkjet printing head in the main scanning direction. It is impossible to make the inkjet printing head compact. Further, when it becomes large in the main scanning direction, the inkjet printing head takes a long time to scan, in the main scanning direction, i.e. a space which is longer than a printing line.
Although it can reduce a space for the nozzles, the bubble jet system has to heat ink to about 400xc2x0 C. It is necessary to prevent the ink from being burnt. Therefore, kinds of usable ink are very limited, and printing quality remain to be improved. Although oil based ink offering good printing quality is available, it cannot be used for the bubble jet printer.
Further, since a vapor pressure of heated ink is used to emit ink, it is not possible to vary the sizes of ink drops easily. This means that it is impossible to gradate printed images by fine-controlling the size of ink drops.
Ink containing a penetrant can shorten a drying period but blurs printed images, which will result in poor printing quality.
The invention is aimed at providing an inkjet printing head for color printing which has a reduced width in a main scanning direction, is compact in size, and assures improved printing quality.
According to a preferred embodiment, there is provided an inkjet printing head in which piezoelectric elements are flexed by a drive voltage applied thereto and cause a diaphragm, where the piezoelectric elements are disposed, to shudder so as to eject ink via nozzles. The inkjet printing head comprising: (a) a plurality of nozzles arranged in a plurality of rows on a head base plate, each of the nozzle rows being arranged in an inclined manner along a sub-scanning direction where a printing medium is moved, and being spaced from an adjacent nozzle row by a distance which is larger than a printing width; and (b) a plurality of pressure chambers disposed on the head base plate, the pressure chambers being in communication with the nozzles, changing their capacity in response to shudders of the diaphragm, and being arranged in two rows in the sub-scanning direction, in the vicinity of each nozzle row.
With the foregoing arrangement, a plurality of nozzle rows are disposed on the head base plate. A printer including this inkjet printing head prints images in colors by ejecting different colors of ink via nozzle rows. The nozzle rows are arranged in an inclined manner along the sub-scanning direction, which prevents the printing head from becoming wider in the main scanning direction. Thus, the inkjet printing head can be made compact. Since the printing head is small in the main scanning direction, it is not required to move unnecessarily in the main scanning direction.
The nozzles are arranged in an inclined manner along the sub-scanning direction, which increases dot density.
The pressure chambers in communication with the nozzles are arranged in two rows in the sub-scanning direction, in the vicinity of the nozzle rows. It is possible to reduce a nozzle pitch in the sub-scanning direction. This two-row arrangement of the pressure chambers can reduce the head width in the main scanning direction.
The nozzle rows are spaced apart by a distance which is larger than a printing width which is covered by one nozzle row. Thus, ink channels and pressure chambers can have sufficient widths in the sub-scanning direction. Further, the piezoelectric elements have sufficient length. Ink can therefore be supplied to the pressure chambers without any difficulty. The ink can be powerfully ejected via the nozzle, so that a printed image has good quality. The nozzle rows are arranged with sufficient space therebetween, so that there is sufficient time from printing in one color till printing in another color, during which ink can be dried completely. Therefore, no penetrant is required. It is possible to offer printed images without blurred portions.